Help making pre-amp kit work with turntable

[caphair]

Hi guys I was handed a late 80s Pioneer turntable that normally gets power from a receiver which handles amplification. I don't have the receiver so I made a power supply for it and attached a Bluetooth transmitter to the audio outs.

I thought my Bluetooth speaker would handle the amplification but I forgot the fact that pre-amps are needed for phono devices so the audio output is very low.

I was looking for cheap kits on eBay and came across a couple headphone pre-amps. Now my question is, are these really "pre-amps"? My understanding is that modern devices like iPods/iPhones etc don't require preamps and just use amplifier circuits so what's the purpose of these kits and would they be applicable for what I'm looking to do?

Here's one in question: http://www.ebay.com/ulk/itm/231606823087

[stj]

Re: Help making pre-amp kit work with turntable

before you get to that point, is the cartridge magnetic, or ceramic?

i'v not dealt with decks for a long time, but there are 2 cartridge types and the output levels are hugely different.
you used to be able to get pre-amp kits to convert decks to magnetic cartridges because they output so much lower!

[SteveNielsen]

Re: Help making pre-amp kit work with turntable

Not only that but the freq response of a normal preamp won't do for a phono preamp, it has to be eq'ed to the old RIAA curve, other wise it's gonna sound like hissy shit.

[caphair]

Re: Help making pre-amp kit work with turntable

Ok I believe it uses a ceramic cartridge. It's a pioneer model PL-555Z

So what should I be looking for in terms of an external preamp?

[SteveNielsen]

Re: Help making pre-amp kit work with turntable

It will have to be a preamp for ceramic phono cartridges. They don't appear to be as common as those for magnetic cartridges but you can NOT use one for a mag with a ceramic, the ceramic high output will overdrive the input all to hell.

There are circuit diagrams on the web if you want to build one. They aren't complicated.

[stj]

Re: Help making pre-amp kit work with turntable

like this.

[budm]

Re: Help making pre-amp kit work with turntable

Are you sure it is CERAMIC cartridge? Pictures of the cartridge?
Those I worked on use AUDIOTECHNICA magnetic cartridge.

[fzabkar]

Re: Help making pre-amp kit work with turntable

An RIAA Preamplifier For Magnetic Cartridges (kit):
http://archive.siliconchip.com.au/cm...6/article.html
http://archive.siliconchip.com.au/cm...=1&slideshow=1

Photos, PCB artwork, circuit diagrams:
http://archive.siliconchip.com.au/st.../30616_4mg.jpg
http://archive.siliconchip.com.au/st.../30616_7mg.jpg
http://archive.siliconchip.com.au/st.../30616_8mg.jpg
http://archive.siliconchip.com.au/st.../30616_9mg.jpg
http://archive.siliconchip.com.au/st.../30616_5mg.jpg
http://archive.siliconchip.com.au/st...30616_11mg.jpg
http://archive.siliconchip.com.au/st.../30616_2mg.jpg
http://archive.siliconchip.com.au/st...30616_10mg.jpg
http://archive.siliconchip.com.au/st.../30616_6mg.jpg

I have an SP-24A (QM51F) stereo phono preamplifier that I purchased around 10 years ago. I still see a few listings on eBay.

[caphair]

Re: Help making pre-amp kit work with turntable

I'll have to double check if it's ceramic or magnetic.

Per researching online the difference my understanding is a ceramic cartridge creates a more flat signal with a higher output where as a magnetic doesn't and requires a preamp for that.

Now does this mean a ceramic cartridge driven turntable doesn't require a preamp to boost to line level going into the amp, or it still does just a different preamp without the function to make the signal flat?

[fzabkar]

Re: Help making pre-amp kit work with turntable

AIUI, a ceramic cartridge can directly drive a regular line input whereas a magnetic cartridge requires amplification and RIAA equalisation. That is the function of a "phono" input.

The following discussion has an excellent explanation:

https://www.hydrogenaud.io/forums/in...howtopic=71491

[caphair]

Re: Help making pre-amp kit work with turntable

Thanks for that link. To quote the final verdict in it:

"Ceramic cartridges do not require a pre-amplifier for RIAA equalization because they respond to record grooves differently. Ceramic cartridges are less sensitive to higher frequencies and progressively reduce their amplitude. The result is a rather imperfect approximation of the RIAA curve."

So a preamp isn't necessary for RIAA equalization, but a preamp without that circuitry is still needed to boost the signal to line level? Am I understanding that correctly?

[fzabkar]

Re: Help making pre-amp kit work with turntable

Ceramic Cartridges and RIAA Equalization:
http://webcache.googleusercontent.co...howtopic=71491

Magnetic cartridges are constant-velocity devices: the signal is proportional to the velocity of the stylus. Since high frequencies cause the stylus to "wiggle" faster, you naturally get an increasing velocity (and hence signal level) as frequency rises. The cutting head used to make the master is also a constant-velocity device, so a magnetic cartridge "matches" the characteristics of the cutting head. The RIAA EQ is applied during cutting in order to restrict excessive groove excursion (bass cut) and to overcome surface noise (treble boost). When you play a record that was cut with a constant-velocity cutter using a constant-velocity cartridge, you therefore need to apply inverse EQ to correct the frequency response.

In contrast, ceramic cartridges are constant-amplitude devices. In other words, the signal level is proportional to the amplitude of the side-to-side motion of the stylus. There is no natural tendency for the signal level to increase at higher frequencies (because the amplitude doesn't get bigger), and hence RIAA EQ should not be applied. Of course, the frequency response characteristic you get out of an unequalised ceramic is only approximately OK-ish. But since its intrinsic sound quality is so low that's an academic point.

Consider an LP groove with a frequency of X. In order for the stylus to trace this groove for a certain time period, it needs to travel a distance proportional to X. Now double the frequency to 2X. The groove wiggles back and forth twice as many times in a given period of time, and therefore the stylus must necessarily travel twice as far. Given that it must do so in the same period of time, it therfore follows that its speed is doubled. And if its output is proportional to velocity (a magnetic cartridge), the signal level also doubles. (Twice the signal level = 6.02dB, and twice the frequency = 1 octave, hence Arny's statements about 6dB/octave). But if its output is proportional to amplitude (a ceramic cartridge), the signal level depends only on the size of the sideways deflection.

The difference between constant-velocity and constant-amplitude is down to simple geometry. I suppose the analysis of geometrical changes with respect to time is the area of mathematics we call "mechanics", so you could use that term. But it's important not to confuse this term with what a lot of people think of when you say "mechanical" (like how machines work).

The reason why a magnetic cartridge is constant-velocity is due to the laws of electromagnetics. If you move a magnet next to a coil, you induce a current that is proportional to the speed at which the magnet moves (all other things such as strength of magnet, turns on the coil, etc. being equal, of course).

A ceramic cartridge is a constant-amplitude device because the nature of the piezo-electric effect is such that the generated voltage is proportional to the pressure applied.

I can't explain it any better than the above quotes.

A ceramic cartridge produces a signal amplitude that is sufficient to drive a standard line input whereas a magnetic cartridge needs to be amplified further.

The signal amplitude of a ceramic cartridge follows the amplitude of the groove, so it does not require any change to its frequency response, ie no RIAA equalisation.

The signal amplitude of a magnetic cartridge is proportional to the frequency of the groove, so it does require changes to its frequency response.

[stj]

Re: Help making pre-amp kit work with turntable

get the make & model of the cartridge.

[caphair]

Re: Help making pre-amp kit work with turntable

Thanks fzabkar for all that. I was under the impression it was a ceramic cartridge thus confused why if such a cartridge works at line level that the audio out was faint.

Found out it does in fact use a magnetic type

[berniedd]

Re: Help making pre-amp kit work with turntable

Be aware that there are two types of magnetic cartridges: moving magnet (the more common, less costly type) and moving coil (more costly, less commonly used on stock ttables). Each needs a preamp tailored for the cartridge type. Yours seems to be the moving magnet type.

[kaboom]

Re: Help making pre-amp kit work with turntable

AIUI, a ceramic cartridge can directly drive a regular line input

They can not.

A ceramic cart appears to the signal as a high series XC. If loaded with anything within the ballpark impedance of typical line inputs (10K-47k), that series reactance/capacitance forms a C-R hipass filter. Bass response is severly reduced.

Ceramic carts need to be loaded a high impedance, on the order of 470K or more. What's required is an impedance matching preamp- a Hi-Z to Lo-Z buffer.

Even the typical circuit from the 70's will do. I've attached part of the schematic from a Sanyo/Fisher 4080. Notice I've added two caps and lines to an external power supply for "stand-alone" use." Use modern devices for the transistors- KSC/2SC1815, KSC945, or even 2SC2240. The 3.3M bias resistors may need to be changed to suit substitute transistors- you want approx 1/2 B+ from the emitters to GND. If the output level is too low, connect the 0.47u coupling caps directly to the emitters instead of the junction of the 6.8K and 2.7K resistors. This will increase the output level to 275mV.